This trial is an open-label pharmacokinetics study in 40 patients who are naive to NNRTI's and who are stable (1 month or longer) on ZDV (or d4T), 3TC and either Indinavir or Nelfinavir. Patients will receive S-1153 at one of two dose levels for a period of 28 days and will be evaluated for pharmacokinetics and side effects throughout the study. A secondary endpoint will measure virologic, immunologic and viral resistance patterns.